Fix PHP 8.1 issue in DifferentialChangeset getOldStatePathVector()

Summary:
The DifferentialChangeset getOldStatePathVector() method assumes oldFile and filename are set.
This worked under PHP <= 8.0, but fails for PHP >= 8.1 with error messsage

strlen(): Passing null to parameter #1 ($string) of type string is deprecated

Fixes T15517

Test Plan:
Create a diff in which a new file is added.
This file will have oldFile NULL and filename a string.
View the diff https://my.phorge.site/D1234

Reviewers: O1 Blessed Committers, Matthew

Reviewed By: O1 Blessed Committers, Matthew

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15517

Differential Revision: https://we.phorge.it/D25323